Micro-electromechanical systems (MEMS) are very small movable mechanical structures built using standard semiconductor processes. MEMS can be arranged to function as actuators, which are useful in many applications. These actuators typically have a length of a few hundreds of microns, and oftentimes a width of only a few tens of microns. A MEMS actuator is usually configured and disposed in a cantilever fashion in that it has an end attached to a substrate and an opposite free end which is movable between at least two positions, one being a neutral position and the others being deflected positions.
One possible use for MEMS actuators is to configure them as switches. These switches may be locking switches, i.e., relays. These switches are made of at least one actuator. In the case of multiple actuators, they are operated in sequence so as to connect or release one of their parts to a similar part on the other, mating actuator. Because they are extremely small, a very large number of MEMS switches can be provided on a single chip.
MEMS switches have many advantages. Among other things, they can be inexpensive and, as noted, very small. Typically, their power consumption is minimal and their response time is extremely short, e.g., the complete sequence for closing or opening a MEMS switch may be only a few milliseconds.
Typical actuators are based on a pair of arms, one being the so-called “cold” arm, the other being the so-called “hot” arm, both being anchored to a layer on the substrate at one end and linked rigidly together at their opposite end. Note that the hot arm is typically an open end narrow wire loop that extends from two anchors at its open end to, at its closed end, about the end of the cold arm. Also, typically, the hot and cold arms are electrically isolated. The switch contact point is typically attached to the cold arm and located in the vicinity of the location at which the beams are linked.
The deflection of the arms is initiated by applying a potential difference between the pair of terminals, called “anchor pads”, which anchors the hot arm to the substrate. This is the control signal for the actuator. The potential difference causes a current flow in the hot arm elevating its temperature by Joule heating. This heating ultimately causes a part of the hot arm to contract or elongate, depending on the material being used. Presently, the materials of choice for the hot arm are nickel alloys due to their large thermal expansion coefficient, reasonable electrical conductivity, and ease of fabrication by electroplating techniques. Given that the cold arm is not intentionally heated, and so should not change in size, the resulting differential in the size of the beams of the hot and cold arms, which were initially the same size before heating, produces a lateral displacement at the end where the arms are linked.
An actuator may have one or more hot arms, as well as one or more cold arms, depending on the design. Such actuators are arranged to form a switch that can be selectively opened and closed.
One exemplary such actuator is disclosed in U.S. Pat. No. 6,407,478 issued to Wood et al. on Jun. 18, 2002, incorporated by reference as if fully set forth herein. Wood et al. also discloses simplified switches and switching arrays that use micro-electromechanical devices that have one or more beam members that are responsive to temperature. For example, a micro-electromechanical device includes first and second beam members that have respective first ends connected to anchors, and that are also connected together, e.g., near their ends opposite to the anchors. By connecting them together, the first and second beam members thus form a loop. The first and second beam members are connected to a dielectric tether by a first tether anchor. The micro-electromechanical device further includes a third beam member that has a first end that is connected to an anchor and that is connected to the dielectric tether by a second tether anchor. At least one of the first and the second beam members are configured to elongate when the first and/or the second beam member is heated to a temperature that is greater than a temperature of the third beam member. Thus, the first and second beam members form the hot arm while the third beam member is the cold arm. By using two beam members to carry a control current to heat one or both of the two beam members, micro-electromechanical devices may electrically isolate a control signal path defined by the first and the second beam members from a load signal path defined by a third beam member. Such actuators are know in the art as a “heatuator”.
A micro-electromechanical latching switch, i.e., a relay, may be formed using a pair of switch contacts attached to a substrate, and first and second actuators as described. The first actuator has a first end that is connected to the substrate, and has a contact connected thereto. The first actuator further includes a first tab that is attached to the contact. The first actuator is operable to deflect in response to an electrical current. The second actuator has a first end that is connected to the substrate and has a second tab that is connected thereto. The second actuator is operable to deflect in response to an electrical current. The first and the second actuators are positioned such that the contact electrically connects the pair of switch contacts when the first tab engages the second tab between the pair of switch contacts and the second tab. Furthermore, the contact does not electrically connect the pair of switch contacts when the second tab engages the first tab between the pair of switch contacts and the first tab. Arrays of such relays may likewise be formed on a single substrate.
See also Agrawal, A Latching MEMS Relay for DC and RF Applications, Published in IEEE Electrical Contacts, 2004, Proceedings of the 50th IEEE Holm Conference on Electrical Contacts and the 22nd International Conference on Electrical Contacts, pages 222-225, 20-23 Sep. 2004 having ISBN: 0-7803-8460-1, INSPEC Accession Number: 8291957, and Digital Object Identifier: 10.1109/HOLM.2004.1353121 and was posted online: 2004-11-08 17:30:48.0, which is also incorporated by reference as if set forth fully herein.
U.S. Pat. No. 6,407,478 issued to Menard et al. on May 2, 2006, which is incorporated by reference as if fully set forth herein, discloses a MEMS cantilever actuator mounted on a substrate, the actuator including an elongated hot arm member having two spaced-apart portions, each provided at one end with a corresponding anchor pad connected to the substrate, the portions being connected together at a common end that is opposite the anchor pads; an elongated cold arm member adjacent and substantially parallel to the hot arm member, the cold arm member having at one end an anchor pad connected to the substrate, and a free end that is opposite the anchor pad thereof; and a dielectric tether attached over the common end of the portions of the hot arm member and the free end of the cold arm member to mechanically couple the hot arm member and the cold arm member and keep them electrically independent.
The various elements of the actuators employed in an array of switches, as well as the signals to be switched, are typically routed in wires of a layer formed on the substrate but below the layers in which the actuators are formed. As a result, the actuators must be formed of materials the processing of which will not destroy the wires. Unfortunately, high temperature processes will destroy such wires. Hence, low temperature processing is typically employed, and thus the hot and cold arms of the actuator are typically formed of metal, e.g., nickel, e.g., using well-known molding and plating techniques, e.g., as described in the Agrawal paper.
Metal has good conductive properties, and hence is suitable to be used as the electrical conductor for signals to be switched by such a switch. Also, the thermal expansion of metal is relatively high, making it deflect strongly upon heating, and hence being very suitable for use as the hot arm. However, disadvantageously, metal suffers from so-called “creep”, meaning failure of the restorative force properties of the metal to return it to its original shape when the force upon the metal that distorted its shape is removed.
By contrast, silicon does not suffer from creep, but it has relatively high resistance, making it unsuitable to be used as the electrical conductor for signals to be switched by such a switch. Furthermore, silicon requires high temperatures for processing to form elements therefrom, such high temperature processing being inimicable to the underlying wires. Lastly, the thermal expansion of silicon is relatively limited as compared to that of metal, in that silicon does not deflect as strongly as a typically employed metal would upon heating, and hence silicon is relatively unsuitable for use as the hot arm.